Liquid crystalline (LC) polymers with cross linked networks are becoming attractive due to possibilities for freezing the LC phase (Shlota, A. and Ober, C. K. Prog. Polym. Sci., 1997, 22, 975; Morman, W. and Zimmermann, J. G., Macromolecules, 1996, 29, 1949; Morman, W., Trends in Polymer Science, 1995, 2(8), 2559; Koner, H. And Ober, C. K., Polym. Mater. Sci. Eng. Prepr., 1995, 73, 456; Morman, W. and Zimmermann, J. G., Macromol Symp., 1995, 93,96; Wang, Y. H., Hong, H. L., Yang, F. S. and Hong, I. L., Polym. Mater. Sci. Eng. Prepr., 1994, 71, 678; Melissaris, A. P., Sutter, J. K., Litt, M. H., Scheiman, D. A. and Scheiman, M., Macromolecules,1995, 28, 860; Hoyt, A. E. and Huang, S. J. J. Macromol. Sci.: Pure Appl. Chem., 1995, A32, 1931; Hikmet, R. A. M., Lub, J. and Tol, A. J. W., Macromolecules, 1995, 28, 3313; Lai, W. W. and Chang, T. C., J. Polym. Sci., Polym. Chem. Ed., 1995, 33, 1075; Douglas, E. P., Langlois, D. A. and Benicewice, B. C., Chem. Mater., 1994, 6, 1295; Mallon, J. J. and Adams, P. M. J. Polym. Sci., Polym. Chem. Ed., 1993, 31, 2249; Barclay, G. G. and Ober, C. K., Prog. Polym. Sci., 1993, 18, 899; Navarro, F., Macromolecules, 1991, 24, 6622; Peter, K. and Ratzsch M., Macromol. Chem., 1990, 191, 1021; Taroze, R. V,, Gubina, T. I., Shibaev, V. P, Plate, N. A., Pakin, V. I., Shamakova, N. A. and Shukov, F., Macromol. Chem. Rapid Commun., 1990, 11, 67; Mitchel, G. R., Davis, F. J. and Ashman, Polymer, 1987, 28, 639; Zentel, R. and Reckert, G., Macromol. Chem., 1986, 187, 1915; Finkelmann, H., Kock H. J. and Rehage, G., Macromol. Chem. Rapid Commun., 1981, 2, 317; Percec, V. (Ed), Liquid Crystalline Polymers, Parts 1-4, Prog. Polym. Sci, 1997, 22). These cross linked polymers pass through a mesophase during curing and retain a mesophase in the final state (Shiota, A. and Ober, C. K. Prog. Polym. Sci., 1997, 22, 975). By combining the ability to lock in an ordered network structure with the ability to macroscopically align the network, these materials can be used for preparation of films and bulk materials where highly ordered structures can be obtained. LC main chain/side chain polymers with cross linked networks can be prepared by chemical/thermal/photo-cross linking of reactive double bonds in the chain or by reaction with a cross linking agent or by copolymerisation (Morman, W. and Zimmermann, J. G., Macromolecules, 1996, 29, 1949; Koner, H. And Ober, C. K., Polym. Mater. Sci. Eng. Prepr., 1995, 73, 456; Wang, Y. H., Hong, H. L., Yang, F. S. and Hong, I. L., Polym. Mater. Sci. Eng. Prepr., 1994, 71, 678). Thus, investigations on a number of rigid rod liquid crystalline thermosets such as biacetylene rigid rod thermosets (Shiota, A. and Ober, C. K., Prog, Polym. Sci., 1997, 22, 975; Melissaris, A. P., Sutter, J. K, Litt, M. H., Scheiman, D. A. and Scheiman, M., Macromolecules,1995, 28, 860; Douglas, E. P., Langlois, D. A. and Benicewice, B. C., Chem. Mater., 1994, 6, 1295), cyanate ester rigid rod thermosets (Barclay, G. G. and Ober, C. K., Prog. Polym. Sci., 1993, 18, 8991, rigid rod epoxy thermosets (Carfagna, C., Amendola, E., Giaberini, M., Filiprov, A. G. and Bauer, R. S., Liq. Cryst., 1993, 13, 571; Dhein, R., Meier, H. -M., Muller, H. -P. And Gipp, R., German Offen 3622613, A1, 1988; Earls, J. D. and Hefner, R. E., Jr., Eur. Patent Appl. 379057, A2, 1990; Hefner, R. E., Jr. and Earls, J. D., Eur. Patent Appl. 475238, A2, 1992; Eirchmeyer, S., Muller, H. -P. and Karbach, A., Eur. Patent Appl. 445401, A2, 1991), bismaleimide rigid rod thermosets (Hoyt, A. E. and Huang, S. J. J. Macromol. Sci,: Pure Appl. Chem., 1995, A32, 1931) and semirigid rod liquid crystalline thermosets (Shiota, A. and Ober, C. K. Prog. Polym. Sci., 1997, 22, 975; Hikmet, R. A. M., Lub, J. and Tol, A. J. W., Macromolecules, 1995, 28, 331) were reported. All these polymers employ the conventional networking using a multiple epoxy grouping or a diacetylene moiety or diacrylate moiety. No report on the formation of cross link through a side chain containing multiple double bonds could be found in patent literature or publications on cross linked liquid crystalline polymers. However, the side chain of cardanyl acrylate has been reported to be used in cross link formation in poly(cardanyl acrylate) which is not liquid crystalline (John, G. and Pillal, C. K. S., Macromol. Chem. Rapid Commun. 1992, 13, 255; John, G. and Pillal, C. K, S., J. Polym. Sci, Polym, Chem., 1993, 31, 1069; Pillai, C. K. S., in Handbook of Engineering Polymeric Materials, (Ed) N. P. Cheremisinoff, Marcel Dekker, New York (1997). Cardanol is a naturally occurring material and hence is inexpensive. The formation of the cross-linked polymer has been explained as due to the autocross linking of the unsaturated side chain of cardanyl unit (John, G. and Pillai, C. K. S., J. Polym. Sci, Polym. Chem., 1993, 31, 1069). This property of the cardanyl side chain can be used to form crossslinked network structures for liquid crystalline polymers. Cardanol was shown to possess special structural features that allow it to be transformed into high performance polymers including liquid crystalline polymers (Pillai, C. K. S., Sherrington, D.C. and Sneddon, A., Polymer, 1992, 33, 3968; Saminatnan, M., Pillai, C. K. S, and Pavithran, C., Macromolecules, 1993, 26, 7103; Pillai, C. K. S., Sherrington, D.C. and Sneddon, Indian Patent Application No. 679/Del/92 dated Jul. 29, 1992; Saminathan, M. Pillai, C. K. S. and Pavithran, Indian Patent Application No. 2791/Del/92 dated Dec. 31, 1992; Saminathan, M., Pillai, C. K. S. and Pavithran, Indian Patent application No. 29721 Del/92 dated Dec. 31, 1992. In an earlier patent (Saminathan, M., Pillai, C. K. S. and Pavithran, C., Indian Patent Application No. 2972/Del/92 dated Dec. 31, 1992), it was shown that a liquid crystalline polymer, poly[4-(hydroxy-2-pentadecyl)azo]benzoic acid] can be prepared from 3-n-pentadecyl phenol, the hydrogenated derivative of cardanol and that this polymer is potentially a non-linear optical (NLO) material (Saminathan, M., Pillai, C. K. S. and Pavithran, C., Macromolecules, 1993, 26, 7103). Azobenzene derivatives and polymers containing donor-acceptor groups in conjugation are known for their NLO activity (Sucdesh Kumar, G., AZO Functional Polymers: Functional Group Approach in Macromolecular Design, Technomic Publications Lancaster 1992; Sutherland, R. L., Handbook of Nonlinear Optics, Marcer Dekker, New York, 1996 and I -C. Khoo, F. Simoni, and C. Umeton, Novel Optical Materials and Applications, Wiley-VCH Publications, Weinheim, Germany, 1996). To retain the NLO property in a polymer, it is, however, important to prevent the dipole reversal or relaxation from their dipolar alignment. One of the methods recommended to arrest the relaxation is by introducing cross links during poling which “lock in” dipole alignment due to the restricted molecular mobility.
Azobenzene derivatives and polymers containing donor-acceptor groups in conjugation are known for their non-linear optical activity. To retain the non-linear optical property in a polymer, it is, however, important to prevent the dipole reversal or relaxation from their dipolar alignment. One of the methods recommended to arrest the relaxation is by introducing cross links during poling which “lock in” dipole alignment due to the restricted molecular mobility.
It is therefore important to obtain azobenzene derivatives capable of providing polymers which have non-linear optical activity.